1. Field of the Invention
This invention relates to fibers, especially polyimide fibers, having a dielectric constant below 3, as well as excellent thermal stability and tensile properties within the range of standard textile fibers. It relates also to a process for producing such fibers by wet spinning polyimide or polyamic acid solutions, especially fluorine-containing aromatic polyimide or polyamic acid solutions.
2. Description of the Related Art
The need for materials having a low dielectric constant (below 3) has grown in the aerospace and electronic industries in recent years. These materials find use as coatings, films, matrix resins, and fibers in various applications.
Several types of fibers have been employed in the construction of low dielectric composites. SPECTRA.RTM., produced by Allied-Signal, has a dielectric constant of 2.35 (data sheet from Allied-Signal). However, a melting point of 147.degree. C. [Chem. & Eng. News, 24, Oct. 9, 1989] prohibits its use in high temperature applications. Silicon carbide fibers, NICALON.TM. from Dow Corning, retain their tensile properties at elevated temperatures, but they have a dielectric constant of 6.4 at 9.4 GHz (Dow Corning product bulletin). Ceramic fibers such as NEXTEL.RTM., from 3M, can be used at elevated temperatures and have a slightly lower dielectric constant of 5.0 at 1 GHz (3M product bulletin). Glass and KEVLAR.RTM. (from Du Pont) fibers are used in high temperature applications and exhibit dielectric constants of 3.6-5.8 and 3.7, respectively [SAMPE Quarterly, Apr. 27, 1979]. Quartz fibers, ASTROQUARTZ.RTM. II from J. P. Stevens & Co., have use temperatures up to 1050.degree. C. and a dielectric constant of 3.78 at 1 MHz (J. P. Stevens Products/Data/Price list). Polybenzimidazole fibers, from Hoechst Celanese Corp., have high temperature resistance (glass transition temp. of 425.degree. C.) and a dielectric constant of 3.3 at 0.1 MHz (Celanese PBI product bulletin). A high temperature resistant fiber with a dielectric constant less than 3 is not reported in the literature.